In the combustion of coal in power plants, nitrogen oxides are formed, which are usually designated by the symbol NOx. These oxides are environmental contaminants and should not be allowed to enter the atmosphere. Therefore, they are largely removed by a process known as nitrogen oxide removal, comprising the reduction of the nitrogen oxide by ammonia as the reducing agent in the presence of a suitable catalyst with the formation of nitrogen. The catalyst, designated as a DeNOx catalyst, generally comprises 50 to 90% by weight of titanium dioxide, 5 to 20% by weight of tungsten trioxide and/or molybdenum trioxide, and 0 to 10% by weight of divanadium pentoxide. Generally, the catalyst has the shape of honeycombs.
There are basically two firing technologies in the combustion of coal: dry firing and molten ash chamber firing. Both firings are generally operated with dustfine ground coal. With dry firing, the combustion temperature is below the fusion temperature of the ash, which results in the fly ash being almost completely entrained by the flue gas stream and separated as fly ash in the electrostatic filter. The fusion temperature of the ash varies with its composition. For example, the fusion temperature of German anthracite coal is about 1400.degree. C.
With molten ash chamber firing, the combustion temperature is above the fusion temperature of the ash, for example, at full load, at about 1500.degree. C. The predominant part of the ash combines into a melt at the bottom of the molten ash chamber. It is conducted into a water bath, where it is quenched. The solidified melt is designated as molten ash chamber granulated material or slag. This material has a low porosity, is glasslike, dark gray to black, and has a particle diameter of less than 15 mm. The slag, comprising primarily aluminum silicate, finds multiple uses in the construction industry. The ash entrained by the flue gas stream is contaminated with arsenic compounds and, after being separated in the electrostatic filter, is dumped as special waste or fed back into the molten ash chamber firing. In the latter case, all the ash from the molten ash chamber firing is obtained as slag which meets environmental requirements and can be utilized in the usual manner. The fly ash from the molten ash chamber firing thus can be utilized without causing a pollution problem.
The DeNOx catalysts used for nitrogen oxide removal, i.e. in the gas firing, oil firing and in case of the combustion of coal in the dry firing and molten ash chamber firing, upon loss of activity, are regenerated or, if this is not possible, are processed for the recovery of the oxides of valuable metals, i.e., of tungsten, molybdenum, and vanadium (DE-OS 35 08 902, EP-OS 0 161 206). The used catalysts are contaminated with fly ash and, in the case of molten ash chamber firing, are contaminated with arsenic compounds. The processes for regeneration and recovery require relatively high chemical and energy consumption, and resultant contaminated residues create a significant waste disposal problem. Even worse, if regeneration and recovery are not possible, the entire untreated, used DeNOx catalysts have to be dumped as waste.
Thus, prior to the present invention, no process was known for the environmentally unobtrusive utilization of used DeNOx catalysts.